Please jump into any conversations about petrology, petrogenesis and mineral paragenesis. Your contribution has been excellent. There are not enough scientific approaches towards mineral and gem ID. I went through a similar conversation along with Troy about a 4 kilogram polycrystalline sapphire, posted first by Kaddah. The sapphire has calcic mica inclusions, indicated by SEM analysis. Margarite is extremely rare in asociation with sapphire. The sapphire’s povenance was unknown. Alkali feldspar and quartz were accessory minerals is the surrounding matrix. This was a pegmatitic metamorphic sapphire within a desilicated plumasite type pegmatite. The addition of calcium into mica makes it even more unusual, Calcium is early crystallizing, the corundum crystallizes later from residual peraluminous fluids. Mg/Gd ratios were not done in order to confirm metamorphic versus igenous sapphire, however probably not necessary in this particular case.
chysoberyl in schist is more rare than pegmatitic. Conditions include medium high to high grade metamorphism (amphibolite to granulite facies). Pelitic sediments- high aluminum must be metamorphosed to provide enough aluminum. Geothermometry and geobarometric indicators are garnet, sillimanite, staurolite, kyanite..the pressure is 0.35 to 0.5 gigapascals, temperature, 500-600 C. Late stage, Berylium bearing fluids create beryl. The breakdown of beryl creates chrysoberyl. (beryl + K feldspar + H+ = chrysoberyl + quartz + K+, H20)..Lower stage metamorphism creates emerald under the same conditions with beryllium… both alexandrite and emerald also require adjacent mafic to ultramafic rock to provide iron, vanadium and chromium. The most common scenario is the intrusion of granitic/pegmatitic rocks into high Al schist, providing for Be. Contact metamorphism between the schist and pegmatitic granite will also create both types of gemstones under different PT conditions. Brazilian and Sri Lankan chrysoberyl is hosted in mafic bands within the aluminous schists. These mafic bands provide the chromium. The juxtaposition of peraluminous conditions and mafic/ultramafic rock is very uncommon, either by formation in schists or in pegmatites. Ancient (neoproterozoic) continental collisions created these conditions in Africa, Sri Lanka and Malagasy. The gem deposits of the Califronian Sierra Nevada were caused by subduction. To the north were orogenic gold deposits, the southern gem pegmatites east of San Diego were enriched by island arc subduction. The most recent large scale orogeny was the collision of India with Asia. To the west in Afghanistan and Pakistan, “sweated out” hydrothermal fluids rich in Be and Li created gemstones when in contact with mafic rock precursors. Desilication of pegmatites by mafic to ultramafic rocks created Kashmir sapphires, tourmalines, beryl, in addition to fault zone bearing metamorphic fluids rich in incompatible light elements- those that do not fit into the cystal structure of common rocks- feldspars, micas. Myamar is unique. The presence of thick limestone beds, metamorphosed into marble, which is stable over a wide range of PT conditions removed silica from incompatible light element fluids. Corundum is present in aluminous metasediments desilicated by the marbles. Chromium without iron was introduced by impure limestone with organic matter and shales in the limestone. Also by intrusions of syenitic to leucograntic rocks and bands of gneiss.